There are 3 services
1) PNP
2) Maldar
3) Ajanta
All above Ferry Services are available from Gateway of India. The journey starts from Gateway of India to Mandwa Jetty. From Mandwa Free Bus shuttle are available which drops you at Alibaug.
Gateway ==(boat)==> Mandwa ==(Bus)==> Alibaug
Alibaug ==(Bus)==> Mandwa ==(boat)==> Gateway